The field of the invention is Computerized Axial Tomography (CAT) scanners, and more particularly, CAT scanners of the type employing a gantry adapted for continuous rotation.
CAT scanners utilize a rotating gantry to obtain multiple X-ray images, or "views", at progressively different rotational angles. Each set of images is referred to in the art as a "slice". A patient is disposed in a central opening of the gantry on a table which is movable axially, thereby enabling slices to be obtained at multiple axial positions as well. All of the slices obtained are then processed in a computer according to known algorithms to produce enhanced images for diagnostic purposes.
The rotating gantry includes an X-ray apparatus and electronics necessary to generate image data for each view. It is therefore necessary to supply electrical power to the rotating gantry to power the electronics, and particularly to provide a high voltage supply for operation of the X-ray tube. Additionally, it is necessary to provide for communication of the image data, and other control information, between the electronics on the rotating gantry and a set of stationary electronics. The stationary electronics then process the raw image data into the enhanced form.
The data rate for communication between the stationary and rotating electronics is an important factor because it is desirable to obtain the desired views as fast as possible to reduce patient discomfort and to maximize equipment utilization. A view typically comprises 730 channels, with a 16 bit representation for each individual channel output (i.e. 11.68K bits per view) and is typically repeated 1,000 times per second, yielding a net data rate requirement of approximately 12 Megabits per second (Mbits/sec) for image data alone.
In order to provide a communications link with the requisite data rate, prior CAT scanners have employed an umbilical cable connected to the rotating gantry. One or more flexible, shielded coaxial cables are used in the umbilical cable for high speed communications, and other conductors are used for power and discrete control signals. The umbilical cable is typically capable of plus or minus 360.degree. of rotation, so that the gantry is limited to a total of 720.degree. of rotation. In operation, the gantry is accelerated to a desired rotational speed and the desired views are taken before the 720.degree. limit is reached. Near the 720.degree. limit, the gantry is decelerated to a stop, and then accelerated in the reverse direction to acquire more views. The gantry thus cycles back and forth within the 720.degree. limit.
Such "cycling" type CAT scanners have two main disadvantages. One disadvantage is that the decelerating and re-accelerating of the gantry is fairly time consuming. The gantry, with all equipment in place, is both large and massive, so that even with large motors, the time consumed in accelerating the gantry can be substantial. The second disadvantage is somewhat of a corollary to the first, in that the need to repeatedly accelerate such a large mass produces a large amount of mechanical stress and wear.
Another type of gantry is known in the art in which brush and slip rings are used for power and communications. In that case, the gantry is free to rotate continuously, eliminating the need for the above described back-and-forth movement of the gantry and thereby providing a greater proportion of time for the acquisition of the desired views. However, prior CAT scanners utilizing brushes and slip rings for communications have suffered a serious limitation of the data rates which can be achieved. The gantry and the slip rings thereon are necessarily both large and unshielded. The slip rings are therefore subjected to substantial electrical noise from external sources. However, the greater problem is in achieving high data rate communications. In that case, a very fast edge rate is needed for the signal applied to the slip rings, while the round trip time on interconnect around the slip rings is much greater than the edge rate, resulting in an unstable oscillatory response. Also, the electrical path length around the rings is an appreciable fraction of a bit period, so that energy propagating around the rings in opposite directions may arrive at a reception point at substantially different times in a bit period, causing garbled reception.
Therefore, a need exists for a high speed communications apparatus for a CAT scanner which provides for both continuous rotation and for reliable, high speed communications.